Soil organic matter is considered to be a major contributing factor to the productivity of organic farming systems. This study was conducted in a semi-arid ecosystem of southern New Mexico with the objective of determining the influence of amount of time under organic farming on carbon accumulation in the soil. We selected four farms: three of them were under organic farming for three, six and nine years since 2005, and the fourth under the conventional farming system. Soil samples were collected during November 2005 and then again during 2006 in triplicate to a depth of 100 cm and were air-dried. Fine ground (<250 um) samples were used for the determination of total C, N and H by dry combustion with a Thermo Electron CHNS-O analyzer. Inorganic C was determined by decomposing carbonates in a sealed serum bottle with 1M HCl and measuring the evolved carbon dioxide by gas chromatography. Organic C was computed as the difference between total and inorganic C.